Electrosurgical forceps for delivering microwave energy from a non-resonant unbalanced lossy transmission line structure

ABSTRACT

Electrosurgical forceps for delivering microwave energy into biological tissue from a non-resonant unbalanced lossy transmission line structure located within or formed by the jaws of the forceps. The transmission line structure may be formed across the gap between the jaw element by opposed conductive elements, which are respectively electrically connected to inner and outer conductors of a coaxial cable. Alternatively, each jaw element may comprise its own lossy transmission line, whereby a power splitter is used to divide microwave energy from the coaxial cable. The forceps may be used endoscopically in the gastrointestinal tract or laparoscopically or in open surgery.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.15/021,936, filed Mar. 14, 2016, which is a National Stage entry ofInternational Application No.: PCT/GB2014/053015, filed Oct. 7, 2014,which claims priority to United Kingdom Patent Application No.1317713.4, filed Oct. 7, 2013. The disclosures of the priorityapplications are hereby incorporated in their entirety by reference.

FIELD OF THE INVENTION

The invention relates to electrosurgical forceps for grasping biologicaltissue and for delivering microwave frequency energy into the graspedtissue to coagulate or cauterise or seal the tissue. In particular, theforceps may be used to seal blood vessels. The forceps may be inserteddown the instrument channel of an endoscope or a gastroscope, or may beused in laparoscopic surgery or open surgery.

BACKGROUND TO THE INVENTION

Forceps capable of delivering heat energy into grasped biological tissueare known. The heat energy may cauterise the grasped tissue andfacilitate coagulation or vessel sealing.

U.S. Pat. No. 6,585,735 describes an endoscopic bipolar forceps in whichthe jaws of the forceps are arranged to conduct bipolar energy throughthe tissue held therebetween.

EP 2 233 098 describes microwave forceps for sealing tissue in which thesealing surfaces of the jaws include one or more microwave antennas forradiating microwave frequency energy into tissue grasped between thejaws of the forceps.

SUMMARY OF THE INVENTION

At its most general, the present invention provides electrosurgicalforceps in which microwave energy is delivered into biological tissuefrom a non-resonant unbalanced lossy transmission line structure locatedwithin or formed by the jaws of the forceps. The forceps may be usedendoscopically in the gastrointestinal tract or laparoscopically or inopen surgery.

According to a first aspect of the invention, there is provided anelectrosurgical forceps comprising: a pair of jaw elements pivotablerelative to each other to open and close a gap therebetween; a firstconductive element mounted in one of the pair of jaw elements adjacentto the gap; a second conductive element mounted in the other one of thepair of jaw elements adjacent to the gap opposite the first conductiveelement; a coaxial cable for conveying microwave energy; and a signaltransition portion at a distal end of the coaxial cable, the signaltransition portion being arranged to connect the first conductiveelement to an outer conductor of the coaxial cable and to connect thesecond conductive element to an inner conductor of the coaxial cable,wherein the first conductive element and the second conductive elementform a non-uniform unbalanced lossy transmission line to support themicrowave energy as a travelling wave, and wherein the first conductiveelement and the second conductive element are non-resonant for themicrowave energy along the travelling wave.

Herein the term “non-resonant” may mean that the electrical length ofthe transmission line (along the microwave energy travelling wave) isset to inhibit multiple reflections of the travelling wave, i.e. toprevent or inhibit the creation of a radiating standing wave. Inpractice this may mean that the electrical length of the transmissionline is substantially different from a multiple of a quarter wavelengthof the microwave energy (an odd or even multiple needs to be avoideddepending on whether the distal end of the transmission line is an opencircuit or a short circuit). It is particularly desirable for thetransmission line to be non-resonant when there is biological tissue inthe gap, i.e. in contact with the jaw elements. Thus, the electricallength of the transmission line may be set to avoid a multiple of aquarter wavelength of the microwave energy when the transmission line isloaded by the biological tissue in this way. Preferably the distal endof the transmission line is an open circuit, as this may enable thedevice to operate with radiofrequency (RF) energy as well as microwaveenergy.

Forming a non-resonant transmission line may prevent the device fromradiating. The microwave energy is therefore delivered into tissuethrough leakage from the transmission line structure. By setting thelength of the transmission line with knowledge of the loss level intobiological tissue at the frequency of the microwave energy, theelectrosurgical forceps of the invention can be arrange to deliversubstantially all of the power received at the proximal end of thetransmission line in a single transit of the travelling wave along thetransmission line.

In other words, the geometry of the transmission line is selected, e.g.on the basis of simulations or the like, such that it exhibits high lossin biological tissue at the frequency of the microwave energy.Similarly, the geometry of the transmission line may ensure that muchless power is lost when there is no tissue in the gap, but air instead.For example, the device may exhibit about 1 dB return loss, i.e. 80% ofpower reflected back to the generator, compared to 20% when there istissue there. Thus, four times as much power can be delivered whentissue is present in the gap. Biological tissue is lossy, i.e. it is agood absorber of microwave energy.

The magnitude of the electric field produced by the forceps of theinvention may be considerably lower than that produced by conventionalbipolar RF forceps. The microwave frequency electric field used in theinvention damages tissue in a fundamentally different way from RFenergy, i.e. by denaturing tissue rather than cell rupture. Thepotential for accidental localised extreme damage is therefore muchsmaller than with RF devices that may generate a plasma or arc and burn.Moreover, the peak voltage required to produce effective dielectricheating with microwave energy may be less than 50 V, which is a factorof 10 lower that required for bipolar RF devices and a factor of 100less than that required for monopolar RF devices. In the latter, thepath for the RF current to flow is through the body via a return plateplaced on the surface of the patient's skin. This presents a risk to thepatient in terms of the high voltage requirement and also lack ofcontrol due to the current always wanting to take the path of leastresistance. It could also cause an explosion to occur within the bodydue to a build-up of gases being ignited due to a spark, arc, microplasma or breakdown occurring because of the high voltage levelsassociated with monopolar RF energy, e.g. 4,500 V peak, or bipolar RFenergy, e.g. 500 V peak or greater. The high voltages associated withbipolar or monopolar RF instruments present as risk of explosion. Incomparison, voltages associated with microwave coagulation may bebetween 5 V and 70 V peak. The device is therefore safer for the patientwhen in use in the environment found inside the patient's body.

The magnitude of the field may be controlled e.g. by controlling thepower delivered to the forceps based on the size of the gap. Thiscontrol may be permit the magnitude of the electric field to beindependent of the size of the vessel or the thickness of the tissuelocated in the gap. This may present an advantage over conventionalbipolar RF forceps.

Herein, “microwave frequency” may be used broadly to indicate afrequency range of 400 MHz to 100 GHz, but preferably the range 1 GHz to60 GHz, more preferably 2.45 GHz to 30 GHz or 5 GHz to 30 GHz. Specificfrequencies that have been considered are: 915 MHz, 2.45 GHz, 3.3 GHz,5.8 GHz, 10 GHz, 14.5 GHz and 24 GHz.

The electrosurgical forceps of the invention may be configured forinsertion down an instrument channel of an endoscope, or may be arrangedfor use in laparoscopic surgery or in a NOTES procedure or in a generalopen procedure.

Herein, the term “non-uniform” transmission line is used to designate anarrangement in which the conductive elements on opposing surfaces of thejaws are not in a uniform spatial relationship with each other along thelength of the pair of jaw elements. For example, the conductive elementsmay comprise a first conductive plate mounted in one of the pair of jawelements and a second conductive plate mounted in the other one of thepair of jaw elements, wherein the signal transition portion is arrangedto connect the first conductive plate to an outer conductor of thecoaxial cable and to connect the second conductive plate to an innerconductor of the coaxial cable. The conductive plates may each include aflat surface at or aligned with the surface of one of the jaw elementsfacing into the gap. This configuration may ensure an optimal powerdensity in the gap between the plates, to ensure that the energy isdelivered into biological tissue present in the gap.

The transmission line may form a parallel transmission line or parallelplate transmission line when the opposing surfaces of the conductiveplates are parallel. However, this is not essential. Over a wide rangeof angles between the jaws, e.g. ±20° or more, the power will travel upbetween the jaws. The jaws might not be parallel for a number ofreasons, such as because they are pivoted at one end or because thetissue held between them is not of the uniform thickness.

Each conductive plate may have a flat elongate structure, e.g. having awidth of 1 to 6 mm and a length of 3 to 12 mm. For endoscopic use, eachplate may have a width of 1 to 3 mm and a length of 3 to 6 mm.Preferably, each plate has identical dimensions. The preferreddimensions may depend on the microwave frequency. Where 5.8 GHz energyis used, the plates may have a width of 2 mm and a length of 4 mm. Theconductive plates may have curved distal ends. Removing sharp cornersmay reduce the risk of bowel wall perforation when operating in the GItract, and may prevent unwanted concentrations of microwave energy. Theconductive plates may have curved proximal ends, e.g. at the point atwhich they connect to the signal transition portion. The thickness ofthe plates may be 0.5 mm or less.

The signal transition portion may include a linking member that extendsfrom a distal end of the coaxial cable, the linking member comprising anextension of the inner conductor of the coaxial cable surrounded by adielectric cover, wherein a distal end of the extension of the innerconductor of the coaxial cable is connected to the second conductiveplate. The linking member may have a length of 3 mm or more. The linkingmember itself may form a non-uniform transmission line.

The signal transition portion may include an outer connector thatextends from the outer conductor of the coaxial cable and electricallyconnects the outer conductor of the coaxial cable to the firstconductive plate. The proximal end of the outer connector may be curvedto wrap around the outer conductor of the coaxial cable. The outerconnector may taper (i.e. decrease in width) as it extends away from theouter conductor of the coaxial cable.

In another aspect of the invention, each jaw element may comprise itsown lossy transmission line. In this arrangement, a power splitter maybe used to divide the power between a pair of transmission lines, one oneach jaw element. Thus, according to a second aspect of the invention,there is provided an electrosurgical forceps comprising: a pair of jawelements pivotable relative to each other to open and close a gaptherebetween; a first transmission line structure mounted in one of thepair of jaw elements adjacent to the gap; a second transmission linestructure mounted in the other one of the pair of jaw elements adjacentto the gap opposite the first transmission line structure; a coaxialcable for conveying microwave frequency energy; and a power splitter ata distal end of the coaxial cable, the power splitter being arranged todivide the microwave frequency energy conveyed by the coaxial cablebetween the first transmission line structure and the secondtransmission line structure, wherein each of the first transmission linestructure and the second transmission line structure consist of anunbalanced lossy transmission line to support the microwave energy as atravelling wave, and wherein each of the first transmission linestructure and the second transmission line structure have an electricallength along the travelling wave that is non-resonant for the microwaveenergy.

Each of the first transmission line structure and the secondtransmission line structure is a parallel transmission line or a coaxialtransmission line. The power splitter may comprise an arrangement offlexible microstrip transmission lines or coaxial transmission lines.For example, the signal transition may comprise any of a Wilkinson powerdivider, an arrangement of two quarter wavelength transformers, a 3 dBpower splitter or the like. If a Wilkinson power divider is used tosplit the power available at the distal end of the coaxial cable intotwo equal parts, then the signal transition may comprise twosemi-circular or straight sections that are each a quarter wavelengthlong at the frequency of operation, i.e. the overall length of thesplitter is a half wavelength at the operating frequency. In thisarrangement the impedance of the transmission lines that form the twosemi-circular or straight sections is set asZ _(W)=√{square root over (2)}Z ₀,

where Z_(W) is the impedance of the line that forms the Wilkinson powerdivider and Z₀ is the characteristic impedance of the of the coaxialcable. In a preferred embodiment, the impedance of the coaxial cable isset to be the same as the transmission line inside the jaws, which inturn is set to be the same as the biological tissue to be treated.

In an arrangement where two quarter wavelength transformers are used, avirtual impedance exists at the proximal end of each quarter wavelengtharm that has a value which is twice the impedance of the characteristicimpedance of the transmission line that feeds this point, i.e. theimpedance ‘seen’ at the end of the transmission line is equal to halfthe value of the virtual impedance. This assumes that the two quarterwave transformer sections are the same impedance, the impedance of thetransmission line inside each of the two jaws is the same, and that eachjaw makes good contact with the biological tissue, which is homogeneousand has a value of impedance that is the same or close to that of theimpedance of the transmission lines inside the two jaws.

A further arrangement could use a transmission line cable with acharacteristic impedance of Z₀ feeding a Wilkinson power divider, whoselines have an impedance of √{square root over (2)}Z₀, where each arm isconnected to a quarter wavelength transformer, which impedance matchesthe characteristic impedance Z₀ to the Impedance of the transmissionlines within the jaws, which is well matched to the impedance of thetissue Z_(t).

The pair of jaw elements may be biased apart, e.g. using springs or thelike. The springs may be made from plastic or other suitable materialthat does not interfere with the manner in which the microwave frequencyenergy is lost between the conductive plates. Alternatively, the jawelements may also be wholly or partially made from memory metal, e.g.Nitinol wire, and be opened and closed based on the application of heatapplied to the structure (wire). This heat may be generated using a DCpower source (resistive heating), which may involve the use ofadditional feed lines, or generated when the microwave field is appliedto the jaws. For the latter, it may be desirable to include, i.e. paintor deposit, a section of lossy material within the jaws or on the jawssuch that some of the microwave field is absorbed by the lossy materialto produce local heat, which causes the jaws to close (or open).

The forceps may be mounted in a cylindrical sheath, i.e. an enclosurefor the coaxial cable and the pair of jaw elements. The sheath may beretractable to expose the pair of jaw elements. The sheath may act as aprotective cover to facilitate insertion of the forceps through theinstrument channel of an endoscope. The diameter of the cylindricalsheath may be less than 2.8 mm.

The forceps may include a jaw closing mechanism in mechanicalcommunication with the pair of jaw elements. For example, the jawclosing mechanism may include a handle and pull trigger in communicationwith the pair of jaw elements via one or more pull wires. The pull wiresmay extend alongside the coaxial cable through the sheath if the forcepsare inserted through an endoscope. In one embodiment, the jaw closingmechanism may include a pantograph arranged to ensure that the jawelements close together in such a way that their surfaces meetsimultaneously along their length.

The pair of jaw elements may be rotatable, e.g. by rotating the sheath.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are discussed below in detail withreference to the accompanying drawings, in which:

FIG. 1 is a modelled structure for an electrosurgical forceps that is anembodiment of the invention;

FIG. 2 is a graph showing return loss for a modelled example of anelectrosurgical forceps that is an embodiment of the invention;

FIG. 3 is a side view of the modelled structure shown in FIG. 1 showingsimulated power loss density in blood;

FIG. 4 is a graph showing return loss for another modelled example of anelectrosurgical forceps that is an embodiment of the invention;

FIG. 5 is a side view of the modelled structure of FIG. 3 showingsimulated power loss density in blood with a smaller spacing between jawelements; and

FIG. 6 is a schematic drawing of an electrosurgical forceps that is anembodiment of the invention;

FIG. 7 is a schematic drawing of an endoscopic microwave forceps that isan embodiment of the invention;

FIG. 8A shows a Wilkinson power divider arrangement that can be realisedusing flexible microstrip transmission line;

FIG. 8B shows a Wilkinson power divider arrangement that may be realisedusing coaxial transmission lines;

FIG. 9A shows a first design for electrosurgical forceps that uses aWilkinson power divider realised using transmission lines;

FIG. 9B shows a second design for electrosurgical forceps that uses aWilkinson power divider realised using transmission lines; and

FIG. 9C shows a third design for electrosurgical forceps that uses aWilkinson power divider realised using transmission lines.

DETAILED DESCRIPTION; FURTHER OPTIONS AND PREFERENCES

The present invention provides a microwave coagulating forceps that canbe operated through the channel of an endoscope. The diameter of thechannel in the endoscope may be 2.2 mm, 2.8 mm or 3.2 mm. The purpose ofthe forceps is to grasp a thin section of tissue and to coagulate orcauterise the portion held between the forceps using microwave energy,e.g. at a frequency of 5.8 GHz or more.

Unlike known microwave forceps, the present invention is not designed toradiate microwave energy. Instead, one or preferably both jaws of theforceps are designed to act as unbalanced lossy transmission lines. Thisfunction may arise from the selected geometry of the jaws.

An advantage of the lossy transmission line structure is that thedelivery of power into tissue can be more accurately focussed in areasof contact with tissue. In known radiating devices, efficient antennafunctionality may be based on contact between the whole antennastructure and the tissue, whereby the absence of tissue contact alongthe whole antenna length can prevent the antenna from operatingefficiently which in turns affects the amount of energy that isdelivered (and reflected back into the instrument). In this design,power will be delivered into the part that is held in the jaw elements,even if it only occupies (touches) part of the jaw elements. Most of theremaining power will be reflected back to the generator rather thanbeing radiated out into the patient. This structure also offers clinicalbenefit in terms of the reflected microwave energy being reflected backalong the jaws of the instrument may be utilised to produce enhancedtissue coagulation as it returns along a path back to the generator. Atthe distal end of the transmission line formed by the two jaws incontact with tissue, a total mismatch occurs due to the fact that theline is either in air or some other mismatched medium at this point,hence the reflection coefficient is unity or close to unity, i.e. all ofthe wave begins to travel back along the jaws back to the source). Thetransmission line structure works on the basis of reflection coefficientor impedance match, whereby if the impedance of the contact tissue iswell matched to the impedance of the transmission line (reflectioncoefficient zero or close to zero). Ideally, all of the energy isabsorbed by the biological tissue that makes contact with the jaws andso all of the energy is dissipated by the time it reaches the distal endof the jaws, hence no energy is reflected back along the transmissionline structure within the jaws back to the generator. Since no resonanceis required for the energy to be delivered, the electrical length of thetransmission line on the jaw element is not constrained in the same wayas it would be e.g. for an antenna. Accordingly, the electrical lengthof the transmission lines used in the present invention may benon-resonant at the frequency of the microwave energy (when thetransmission line is loaded by biological tissue), i.e. not a multipleof a quarter loaded wavelength of the microwave energy.

The power delivered into the biological tissue at any point is given by:P _(t) =P _(i)(1−Γ²),

where P_(t) is the power transmitted into the tissue at a particularpoint, P_(i) is the incident power at the point where the transmissionline makes contact with the tissue load, and Γ is the reflectioncoefficient at that point, which is related to the impedance of thetransmission line (Z₀) and the impedance of the tissue load (Z_(L)) by

$\Gamma = {\frac{Z_{L} - Z_{0}}{Z_{L} + Z_{0}}.}$

The present invention may find particular use in the polypectomyprocedures in the gastrointestinal (GI) tract, where the stem of a polypneeds to be sealed and cut. In such situations, the stem of the polypmay not contact all of the jaws of the forceps.

FIG. 1 shows a basic representative design for a microwave coagulatingforceps that is an embodiment of the invention. The design is a modelcreated using CST Microwave Studio®, which was then used to simulate theperformance as various modifications were made to the structure tooptimise the return loss and power density in biological tissue.

Although the examples below discuss the use of the forceps in anendoscope, the present invention need not be limited in this way. It maybe applicable to laparoscopic techniques or used in open surgery.

FIG. 1 shows a pair of microwave coagulating forceps 100 that is anembodiment of the invention. The forceps 100 comprise a coaxial cable102 for conveying microwave energy from a suitable generator (not shown)down an endoscope instrument channel to a pair of jaw elements 104, 106.The generator may be any device capable of delivering a controllable andstable microwave signal. For example, the apparatus disclosed in WO2012/076844 may be used.

The coaxial cable 102 may be about 1.2 mm or 2.2 mm in diameter in orderto allow room for a jaw operation mechanism in the instrument channel ofthe endoscope. Sucoform 47 manufactured by Huber+Suhner is a suitablecable that is 1.2 mm in diameter and is flexible enough to allow fullmanipulation of the endoscope with the cable within its channel.

In this embodiment, the jaw elements 104, 106 of the forceps aremodelled as two conductive (e.g. metal) plates 0.5 mm thick and 2 mmwide with curved front and back ends. A first jaw element 104 iselectrically connected to the outer conductor 108 of the coaxial cable102 via an angled tapered connector 110. A second jaw element 106 iselectrically connected to the inner conductor (not shown) of the coaxialcable by a linking member 112, which is an extension of the innerconductor and the dielectric 114 that surrounds it beyond the end of theouter conductor 108.

The jaw elements 104, 106 are movable relative to each other to open andclose the gap between them. For example, the jaw elements 104, 106 maybe connected to a hinge or pivot (not shown). The forceps 100 may thusinclude a jaw operation mechanism, which provided mechanicalcommunication between the jaw elements and the distal end of the device.For example, the jaw operation mechanism may comprise one or more pullwires extending alongside the coaxial cable 102 through the instrumentchannel of the endoscope. Such jaw operation mechanisms are well known.In other embodiments, a pantograph arrangement may be used to open andclose the jaw elements in such a way that their surfaces simultaneouslymeet all the way along their length.

The jaw elements 104, 106 may have a maximum separation of 2 mm, e.g.set by a stopper on the hinge. When pressed together, the jaw elements104, 106 would present a distal cross-section area that measures 2.23 mmacross the diagonal. This is small enough for an outer sheath (notshown) to fit around the jaw elements and still allow passage throughthe instrument channel of the endoscope. The sheath may act to protectthe forceps (e.g. from damage or contamination) as it is inserted downthe instrument channel of the endoscope, or to stop snagging or otherdamage as the tool is manipulated into position inside the patient. Thesheath may be torque stable to assist rotation of the forceps. Thesheath may be retractable to expose the jaw elements when the forcepsare in position of use. Alternatively, the forceps may be extendable toprotrude beyond the end of the sheath. In practice it is possible forthe conductive plates that form the jaw elements to be thinner, e.g. 0.4mm or less, as long as they retain sufficient stiffness to preventunwanted flexing in use.

According to the invention, the function of the conductive plates is asunbalanced lossy transmission lines, whereby microwave frequency energydelivered to the jaw element leaks out into the surrounding environment.To optimise the geometry of the blades, the return loss of the modelledstructure was simulated whilst varying a number of parameters, as shownin Table 1.

TABLE 1 Parameters varied simulations Wire length Plate length Jaw widthRun No. (mm) (mm) (mm) Other 0 9 5 2 1 6 5 2 2 4 5 2 3 3 to 5 5 2 4 4 3to 6 2 5 3 4 2 6 3 4 2 With balun 7 3 4 0.75: to 3.25 8 3 4 2 9 3 4 2Gap 1 mm

The parameter wire length corresponds to the length of the wire joiningthe coaxial cable to the conductive plates, e.g. the length of theconnector 110 and linking member 112. In practice it is desirable forthe wires to have a similar length. Any difference in length should be asmall part, e.g. less than one eighth of a wavelength at the operatingmicrowave frequency. It was found that with 3 mm wire length and 4 mmblade length the return loss was better than 7 dB at 5.8 GHz, as shownin FIG. 2. This means that less than 20% of the power is reflected backtowards the generator, and over 80% is available for use at theconductive plates. This is reasonable efficiency, as any improvementscould only increase the power available at the plates by less than 25%.

FIG. 3 shows the results of simulating power absorption in the regionbetween the jaw elements for a 3 mm wire length and a 4 mm plate lengthwhen biological tissue (in this case blood) is present in that region.The power loss density differs between end regions 116 at the distal andproximal ends of the plates and a central region 114.

In the central region 114 the power loss density is about 65 dBW/m³ for1 W input power. In practice, it is expected that the device would beused with an input power of 10 W, whereby the power loss density(heating power) in this region would be 15 dBW/cm³. This is about 30W/cm³ which is enough to raise the temperature of blood by about 7 Ks⁻¹,assuming that the specific heat capacity of tissue is about 4.2 J/g/K,and that the density of tissue is about 1 g/cm³ so that the heatcapacity of tissue is about 4.2 J/cm³/K.

In the end regions 116, the heating rate will be about three times this,i.e. 20 Ks⁻¹.

In this example, the volume of the region between the plates is 4 mmlong by 2 mm wide and 2 mm high, i.e. 16 mm³. The average power densityis about 90 W/cm³, so the total power absorbed in this region is about1.5 W. It is expected that blood or tissue that intrudes into thetriangular gap where the connector 110 and linking member 112 flare outtowards the plates would also be heated.

FIG. 4 shows the return loss when the separation of the plates isreduced to 1 mm. The return loss at 5.8 GHz changes from just more than7 dB to just more than 6 dB. But despite this change, more than 75% ofthe incident power is available to heat the tissue.

FIG. 5 shows the results of simulating power absorption in the regionbetween the jaw elements for the smaller plate spacing, and it can beseen that the power loss density is higher, which might be expectedbecause the slightly lower total power is concentrated in half thetissue thickness. The indicated power density in a central region 118 ofthe gap for 1 W incident power is about 66 dBW/m³, which corresponds toabout 38 W/cm³ for 10 W incident power, which corresponds to atemperature rise of about 9 Ks⁻¹.

In a practical device, the jaw elements may be biased apart, e.g. usingsprings or the like. Such springs may be made from plastic, which willnot affect the results of the simulations discussed above.

The shape of the connector 110 can be optimised to improve the transferof microwave energy to the jaw elements 104, 106. In particular, it isdesirable to hollow out the proximal end of the connector 110 at thecoaxial cable 112 so that it curves around the dielectric 112. Thisgeometry improves the return loss by making more gradual the change fromthe coaxial transmission line of the coaxial cable 112 to the twintransmission lines of the jaw elements 104, 106.

FIG. 6 shows a schematic view of an endoscopic microwave forceps 300that is an embodiment of the invention. The forceps 300 comprises a body308 having a flexible feed cable 306 extending from it. The feed cable306 is not drawn to scale; it has a length and diameter suitable forinsertion down the instrument channel of an endoscope (not shown). Thiscable may be less than 2.8 mm in overall diameter to allow it to beinserted down the instrument channel of an endoscope or a gastroscope.The feed cable 306 comprises a outer sleeve that contains the coaxialcable and jaw opening mechanism discussed above. At a distal end of thecable 306 are a pair of jaw elements 302, 304, which are pivotablerelative to each other about a hinge 305 to open and close a spacebetween opposing surfaces thereof under the control of the jaw openingmechanism.

The body 308 includes a handle 310 and pull trigger 312 which operatesthe jaw closing mechanism in a conventional manner. The pull trigger 312may alternatively be a mechanical slider or any other suitable mechanismthat allow the jaws to be opened and closed. The body 308 is connectedto a microwave signal generator (not shown) by a suitable cable 314.

The geometry of the jaw elements 302, 304 is selected so that theyfunction as lossy transmission lines as discussed above.

FIG. 7 shows another embodiment of the invention, where the microwaveforceps are inserted through the instrument channel 402 of an endoscope400. The proximal end of the feed cable 404 terminates at a handle 406,which includes a pull trigger 408 for operating the jaw mechanism asdiscussed above. A hand grip 410 is clamped onto the feed cable toprovide a means of rotating cable, and therefore controlling theorientation of the jaws 412 at the distal end of the cable. The outersleeve of the feed cable may include internal braids which providetorque stability, i.e. resist twisting of the sleeve relative to thecoaxial cable. Ideally, the translation between rotation of the handleat the proximal end of the device and the circular movement of the jawsat the distal end will be 1:1, but lesser translation ratios, e.g. 1:2may be sufficient.

FIG. 8A shows a first configuration of a Wilkinson power divider 500,which functions to split an input power P1 into two equal parts (P2 andP3) using two quarter wavelength semi-circular lines or arms. Each armmay also function as an impedance transformer. Thus, the completephysical length of the structure is a half of the electrical wavelengthat the frequency of operation. In order for this power splitting designto be used in practice, it may be preferable for the structure to befabricated onto a flexible microwave substrate, where the tracks may beprinted or photo etched. In order to balance the two output ports (P2and P3), it is preferable to include a balancing resistor 502; theimpedance value of this balancing resistor should preferably be twicethe characteristic impedance.

FIG. 8B gives a second configuration of a Wilkinson power divider 600.In this configuration, coaxial lines 602, 604 are used to realise thedivider. If standard 75Ω coaxial cable is used for the quarterwavelength sections 602, 604, the divider will provide a reasonablematch for 50Ω input and output ports. Ideally, if the input and outputports are 50Ω, then the impedance of each of the quarter wavelength armsis 70.71Ω (=√{square root over (2)}×50). In practice, the coaxialimpedance transformer should be as small and as flexible as possible inorder to fit in the endoscope.

FIG. 9A shows a schematic outline for a first example microwave forcepsdevice 700 that uses a Wilkinson power divider 702, where the impedanceZ_(t) of the biological tissue 704 at the frequency of operation is thesame as the impedance of the transmission line 706 within the jaws andis also the same as the impedance of the coaxial cable 708 that connectsthe microwave energy generator to the device. In FIG. 9A a quarter wavetransformer 710 is used at the proximal end between the output of thegenerator 712 and the coaxial cable 708 to match the output impedanceZ_(s) of the generator 712 to the impedance of the coaxial cable 708(which in this embodiment is also the impedance of the biological tissue704 and the impedance of the transmission line 706 within the jaws). Theimpedance of the quarter wavelength transformer 710 is set as √{squareroot over (Z_(s)×Z_(t))}. Normally, the output impedance of themicrowave energy generator 712 will be 50Ω and if it is assumed that theimpedance of blood is 25Ω at the preferred frequency of operation, thenthe impedance of the quarter wavelength transformer 710 will need to be35.36Ω. This transformer could be realised in practice using a standard50Ω co-axial transmission line with the diameter of the inner conductorincreased, the inner diameter of the outer conductor reduced, the valueof relative permittivity (dielectric constant) of the material thatseparates the inner and outer conductors increased, or by varying acombination of these parameters. It would be relatively straightforwardto manufacture 25106 co-axial transmission line.

FIG. 9B shows a schematic outline for a second example microwave forcepsdevice 800 that uses a Wilkinson power divider 802, where the impedanceZ₀ of the coaxial cable 808 that connects the generator 812 to theinstrument is the same as the output impedance Z_(s) of the generator,which is nominally 50Ω. In FIG. 9B there is a quarter wavelengthtransformer 810, 811 located between the distal end of each arm of theWilkinson power divider 802 and the proximal end of a respectivetransmission line 806 that couples to the biological tissue 804. In thisexample, if it is assumed that the impedance Z_(t) of the biologicaltissue is well matched to the impedance of the transmission lines 806within the jaws, then the impedance of the quarter wavelength matchingtransformers 810 is √{square root over (Z₀×Z_(t))}.

FIG. 9C shows a schematic outline for a third example microwave forcepsdevice 900 that uses a pair of quarter wavelength transmission linetransformer sections 902, 903 to match the impedance of the biologicaltissue 904 to the impedance of the coaxial cable 908 and the microwaveenergy generator 912 to ensure efficient power transfer between thegenerator and the tissue load. Again, in this configuration it isassumed that the impedance of the transmission line 906 within the jawsis well matched to the impedance of the biological tissue Z_(t). In thisarrangement, each transformer 902, 903 transforms the impedance ‘seen’at the jaws to a virtual impedance that has a value equal to twice thecharacteristic impedance of the coaxial cable 908, such that theproximal end of the two arms of the transformers are connected inparallel to give an impedance that is equal to the characteristicimpedance of the feed cable (main microwave transmission line). Theimpedance Z₀ of the coaxial cable 908 may be the same as the outputimpedance Z_(s) of the generator 912, and so the function of the twoquarter wavelength impedance transformers is to match Z₀ to Z_(t). Itmay also be noted that the two transformers are connected in parallel atthis point, therefore, the impedance seen at the proximal end of each ofthe quarter wavelength impedance transformers is 2Z₀. Thus, theimpedance of the quarter wavelength matching sections 902, 903 is√{square root over (2Z₀×Z_(t))}.

The invention claimed is:
 1. Electrosurgical forceps comprising: a pairof jaw elements pivotable relative to each other to open and close a gaptherebetween; a first transmission line structure mounted in one of thepair of jaw elements adjacent to the gap; a second transmission linestructure mounted in the other one of the pair of jaw elements adjacentto the gap opposite the first transmission line structure; a coaxialcable for conveying microwave frequency energy; and a power splitter ata distal end of the coaxial cable, the power splitter being arranged todivide the microwave frequency energy conveyed by the coaxial cablebetween the first transmission line structure and the secondtransmission line structure, wherein each of the first transmission linestructure and the second transmission line structure consist of anunbalanced lossy transmission line to support the microwave energy as atravelling wave, and wherein each of the first transmission linestructure and the second transmission line structure have an electricallength along the travelling wave that is non-resonant for the microwaveenergy.
 2. Electrosurgical forceps according to claim 1, wherein each ofthe first transmission line structure and the second transmission linestructure is a parallel transmission line.
 3. Electrosurgical forcepsaccording to claim 1, wherein each of the first transmission linestructure and the second transmission line structure is a coaxialtransmission line.
 4. Electrosurgical forceps according to claim 1,wherein the power splitter is a Wilkinson power divider. 5.Electrosurgical forceps according to claim 4, wherein the Wilkinsonpower splitter is fabricated using flexible microstrip transmissionlines or coaxial transmission lines.
 6. Electrosurgical forcepsaccording to claim 1, wherein the power splitter comprises a pair ofquarter wavelength transmission lines arranged to match a characteristicimpedance of the coaxial cable to an impedance of the first transmissionline structure and second transmission line structure. 7.Electrosurgical forceps according to claim 6, wherein the characteristicimpedance of the coaxial cable is set to a predetermined impedancecorresponding to a biological tissue to be treated, and an impedance ofeach quarter wavelength arm of the power splitter is equal to √2 timesthe predetermined impedance.
 8. Electrosurgical forceps according toclaim 1, including a quarter wavelength impedance transformer at aproximal end of the coaxial cable, the quarter wavelength impedancetransformer being arranged to match an impedance of the coaxial cable toa generator for delivering the microwave frequency energy into thecoaxial cable.
 9. Electrosurgical forceps according to claim 1, whereinthe pair of jaw elements are biased apart.
 10. Electrosurgical forcepsaccording to claim 1, including a sheath for enclosing the coaxial cableand the pair of jaw elements, wherein the sheath is retractable toexpose the pair of jaw elements.
 11. Electrosurgical forceps accordingto claim 10, wherein the sheath is cylindrical and has a diameter lessthan 2.8 mm.
 12. Electrosurgical forceps according to claim 10,including a handle clamped around a proximal end of the sheath fortransmitting rotational motion to the sheath.
 13. Electrosurgicalforceps according to claim 1, including a jaw closing mechanism inmechanical communication with the pair of jaw elements. 14.Electrosurgical forceps according to claim 13, wherein the jaw closingmechanism includes a pantograph arranged to ensure that the pair of jawelements close together in such a way that their surfaces meetsimultaneously along their length.